In the chemical industry iodide and/or iodide compounds have been proposed in numerous applications for use as catalyst and/or catalytic promoters or co-promoters for the production of numerous higher value organic materials from lower valued organic materials. For example, in U.S. Pat. No. 3,634,531, a process for the oxydehyrogenation of ethylbenzene to styrene utilizing an iodide or bromine catalyst is disclosed. According to the specification the principal problem associated with this route to styrene is the presence of 30-50 wt. ppm of iodide in the form of organic iodide compounds in the crude styrene product. It is furthermore observed that the crude styrene product is contaminated with detrimental amounts of these iodide compounds to the extent that when they are subsequently polymerized or co-polymerized discolored products are formed which are not acceptable commercially. In addition, it is noted that the trace amounts of alkyl iodide contaminants in the crude styrene caused deactivation of catalysts in the subsequent polymerization processing steps. Attempts to separate these contaminated iodide compounds from the crude styrene by fractionation have been unsuccessful primarily because the iodide compounds are unstable and split off from the heavy materials from the bottom of the column and form detrimental amounts of iodine which are carried into the overhead product of the column and thereby contaminate the styrene once again by further reaction therewith to once again reform alkyl iodide compounds.
U.S. Pat. No. 3,658,467 addresses the problem of removing iodide compounds from gaseous streams associated with a normal or abnormal operation of atomic energy reactors. In particular, the problem addressed by the '467 patent is the removal of radioactive iodide-containing materials from the reactor environment either during the normal reactor operations or in the event of a fuel element cladding failure whereby radioactive methyl iodides are formed in significant amounts. According to the teachings of this '467 patent the removal of radioactive amounts of iodine gas has been adequately solved by means of the use of carbon filters coated with metals that react with iodine. In particular, charcoal impregnated with silver and/or copper are suggested in the teachings but it is pointed out that although charcoal absorbs elemental iodine at relatively low temperatures it is not particularly selected for the adsorption of organic iodide compounds. The solution proposed in this '467 patent is the use of a zeolite X molecular sieve exchanged with silver operated under gas phase conditions. In contrast, the present invention utilizes liquid-phase conditions and avoids the use of zeolite X in view of the fact that it is a low silica-containing material typically containing only up to 3 moles of silica per mole of alumina in its framework and thus susceptible to attack in organic media such as acidic acid that tends to react with the alumina portion of the framework X zeolite.
In U.S. Pat. No. 4,735,786, another solution to the problem of removing iodine and organic iodide-containing compounds is set forth, but once again, it is focused specifically on the problems associated with radioactive iodide compound discharge from nuclear facilities operated under normal or abnormal conditions. The '786 patent recognizes the deficiencies of the silver-exchange zeolite X adsorbent used in the prior art for gaseous phase adsorption of iodide compounds and points out that under high humidity conditions the capacity of the adsorbent is significantly reduced. The solution taught by the '786 patent is to switch to a different type of zeolite which is characterized as a high silica to alumina pentasil zeolite which has a specified molecular formula. It is well known, of course, that these pentasil zeolites are well represented by the ZSM-5 type of zeolites. As is clearly shown in U.S. Pat. No. 3,702,886 these pentasil zeolites are well known to be of medium pore consideration having pores made up of ten member rings that have dimensions in the range of 5.1 to 5.6 Angstroms. This '786 patent is however silent as to a use of the zeolitic adsorbent disclosed therein in liquid-phase treating of organic streams that have corrosive properties and also fails to suggest a use of large pore (i.e. having 12 member ring openings) silica-rich to alumina zeolitic materials for this service. In addition, the teachings of the '786 patent are focused only on separation of methyl iodide from a gas stream as is clearly shown in the results reported in the examples. As such it fails to suggest any solution to the more difficult problem of removing higher molecular weight alkyl iodides such as the C.sub.6 alkyl iodide material from a corrosive liquid such as acetic acid that is of particular interest to the present invention.
In U.S. Pat. No. 4,913,850, the problem of methyl iodide removal from gaseous stream is further elaborated on and a solution is proposed in which a so called "binderless" zeolite material is utilized which is made up primarily of 80-90% zeolite X and 10-20% zeolite A. Once again however the teachings of the '850 patent teach away from utilizing a silica-rich and preferably large pore zeolite in bound form for liquid-phase treatment of corrosive organic liquids that contain alkyl iodide and other materials of molecular weight much beyond methyl iodide, as is shown in the Example, neither zeolite X or zeolite A performs in an acceptable manner in this liquid phase service.
In U.S. Pat. No. 5,075,084, the progress of the technology for cleaning up gaseous streams containing radioactive methyl iodide is continued by focusing on the unexpected and detrimental recombination reaction that can be induced by the silver-exchange zeolitic adsorbent that is proposed to be used therein. The particular problem addressed and solved in the '084 patent is, more specifically, the problem of the silver-exchanged zeolite material catalyzing the recombination reaction of hydrogen and oxygen which is very exothermic and can cause undesired catalytic ignition of hydrogen with resulting detrimental consequences to the building containing the atomic reactor. According to the '084 patent this undesired side reaction is suppressed by adding a heavy metal such as lead to the silver exchange adsorbent in order to inhibit this side reaction which can cause an exotherm in the pure silver-exchange zeolite as is clearly shown in Example 3. This '084 patent is not particularly enlightening on any of the problems addressed by the present invention and once again fails to point to a silica-rich zeolite for use in clean-up of a corrosive liquid organic medium and only suggests a mixture of zeolite X and zeolite A for use as the adsorbent both of which, as is demonstrated in the Examples, are not acceptable for use in the present invention.
In U.S. Pat. No. 4,088,737, the problem of removal of radioactive methyl iodide from a gas stream is further addressed in a multi-step treatment procedures wherein the initial gas purification treatment step is with a silver-exchanged zeolite exemplified by zeolite X. After iodide breakthrough, the regeneration and concentration steps involves withdrawing the iodide loaded adsorbent from contact with the gaseous, subjecting it to desorption conditions with an H.sub.2 -rich stream to produce a hydrogen-iodide containing off-gas stream and treatment of the iodide-containing off gas stream with a lead exchanged zeolite to concentrate the desorbed hydrogen iodide. Lead exchanged X is specifically exemplified. The advantage of the multi-step treatment is that the storage of the contaminated material is less expensive in a lead exchanged zeolite. Once again the teachings of the '737 patent are not relevant to the solution to the problem addressed by the present inventors in that the medium to be treated as a gas stream and the material utilized in the treatment is a low silica to alumina X-type of zeolite which has not performed satisfactorily in the corrosive environment typically faced by the present invention.
In U.S. Pat. No. 4,615,806, the particular problem of interest to the present invention is well defined in that a corrosive organic, non-aqueous liquid medium is contaminated with undesired quantities of alkyl iodides and is subjected to treatment in the liquid-phase to remove substantially all of the undesired iodide contaminants. As is explained in the teachings of this '806 patent the principal area of concern here is in the manufacture of carboxylic acids such as acetic acid via a process that results in a product stream contaminated with small amounts of iodide compounds such as methyl iodide, sodium iodide, hydrogen iodide and hexyl iodide. These organic iodide contaminants are known to cause processing difficulties in subsequent chemical conversion operations and may contaminate other materials to which the contaminated acetic acid stream is added. After reviewing the prior art on iodide contaminant clean-up the '806 patent focused on using a special type of an organic resin as an adsorbent in solving the problem and in fact taught away from the use of inorganic material of an adsorbent in its Example III. The amount of iodide contaminant in the acetic acid feed stream subjected to clean-up procedure of the '806 patent is indicated as usually between 1 ppb (i.e. part per billion) up to 100 ppb.
Except for the teachings of U.S. Pat. No. 4,615,806, the problem of removing detrimental organic iodide compounds from liquid solutions is unfortunately not as well developed as that associated with removal of these materials from gas streams. The specific prior art associated with removing organic iodides from aqueous or organic liquid solutions is well summarized in U.S. Pat. No. 4,615,806 which points to prior art teachings associated with the use of "gel-type" resins exchanged with iodide-reactive metals such as silver is acknowledged and distinguished. In this '806 patent a new method for removing organic iodides from organic solutions is taught which essentially relies for novelty on the concept of using a macroreticulated strong acid cation-exchange resin which is "stable" in the organic medium that bears the organic iodide compound to be removed and furthermore has at least one percent of its active sites converted to silver or mercury presumably by cation-exchange. The use of macroreticulated resins is stated in this '806 patent as being an advance over the prior art which are generally characterized as gel-type ion-exchange resins. While the invention of the '806 patent has been practiced commercially with some success it suffers from the fact that the resin is essentially an organic material that is known to "swell" or change dimensions (i.e. up to 50% change is permitted) when exposed to an organic medium making adsorbent bed design difficult. It is also vulnerable to decomposition at relatively mild conditions and furthermore is susceptible to chemical attack by the reagents in the corrosive organic liquid that is frequently found to be the vehicle in which the contaminated organic iodide is dissolved. Another disadvantage associated with the use of resins as an adsorbent for removing iodide-containing materials from organic solutions is the fact that the operating temperature for these materials is limited by the temperature at which the resins starts to decompose or undergoes detrimental structural changes due to softening and loss of strength associated with exposure to high temperatures. Typically, resins begin to decompose at relatively low temperatures of approximately 100-200.degree. C. via mechanisms that are associated with destruction of the fundamental network associated with the resin as well as the cation-exchange sites. For example, the resin preferably utilized in the '806 patent is a "strong acid" cation-exchange type of resin which is essentially constructed out of a sulfonated copolymer of styrene and divinylbenzene. At relatively low temperature conditions the acid-exchange sites on this type of resin are well known to be susceptible to an acid catalyzed desulfonation reaction which can result in the release of metal cations into the effluent from the treatment step as well as detrimental sulfur-containing compounds. These materials in turn can interfere with downstream reactions in which the effluent from the iodide-treating step is used for further chemical processing. A careful reading of the '806 patent has not revealed any regeneration or reactivation mechanism taught therein primarily because these steps undoubtedly would have to be performed at relatively high temperatures that the macroreticulated resin taught therein cannot stand without substantial degradation.
The problem therefore addressed by the present invention is to provide an inorganic adsorbent for use in the treatment service specified in the '806 patent which inorganic material will be free of the substantial temperature restrictions, chemical exposure restrictions and physical swelling problems associated with the typical organic materials used in the prior art.
There are significant teachings in the prior art associated with the use of zeolitic-type adsorbents that point away from their utility in this treatment service. In particular, reference may be had to the comparative example recited in U.S. Pat. No. 4,615,806 (at column 6, line 36 through line 49) where a silver-exchanged zeolite, characterized as 1/16 inch Linde 5A pellets, was tested in iodide compound removal service from a corrosive acetic acid liquid stream and found to be unstable in that silver leached from the adsorbent continuously throughout the run and a yellowish precipitate was found in the treated effluent. Given this discouraging result with a zeolitic materials it is indeed remarkable that we have now found that a suitable inorganic adsorbent for use in this corrosive environment, which is accurately described in the '806 patent, is in fact a zeolitic molecular sieve that has been exchanged with a suitable cation material that is known to react with iodide and iodide-containing compounds at relatively mild conditions provided that the zeolitic material is a silica-rich material which is bound with a substantially insoluble binder material. Our findings are, more specifically, associated with the fact that we have now discerned that a suitable inorganic solution to the problem articulated above is the use of an adsorbent containing a zeolitic molecular sieve having a silica to alumina ratio above the point where decomposition of the zeolite is observed to occur in the corrosive organic liquid being treated. By referring to the silica to alumina ratio associated with the zeolite it is of course intended to refer to the framework silica to alumina ratio which is characteristic of the fundamental three dimensional structure which characterizes the zeolite. We have further found that the framework silica to alumina ratio which enables zeolites to perform satisfactorily in this iodide removal surface in a corrosive organic environment such as the presence of acetic acid is not a strong function of the type of zeolite but is rather largely dependent just on the framework ratio provided that it is high enough. In short, we have found good results with framework ratios above about 5:1 with better results found at a silica to alumina ratio of 6.5:1 and with superior results found at a framework ratio of silica to alumina ratio of 8:1. We have unexpectedly found moreover, that such a silica-rich zeolitic adsorbent can be reactivated via a relatively simple procedure and also regenerated at high temperatures when needed.